1. Field of the Invention
The invention relates to circuit interrupters having means for electronically analyzing the electrical conditions on the circuit being protected and for automatically opening to interrupt the current flow whenever electrical conditions exceed predetermined limits.
2. Description of the Prior Art
Circuit breakers are widely used in industrial and commercial applications for protecting electrical conductors and apparatus connected thereto from damage due to excessive current flow. Although initially used as direct replacements for fuses, circuit breakers were gradually called upon to provide more sophisticated types of protection other than merely interrupting the circuit when the current flow exceeded a certain level. More elaborate time-current trip characteristics were required such that a circuit breaker would rapidly open upon very high overload conditions but would delay interruption upon detection of lower overload currents, the delay time being roughly inversely proportional to the degree of overload. Additionally, circuit breakers were called upon to interrupt upon the detection of ground fault currents. As the complexity of electrical distribution circuits increased, the control portions of circuit breakers were interconnected to provide selectivity and coordination. This allowed the designer to specify the order in which the various circuit breakers would interrupt under specified fault conditions.
During the late 1960's, solid state electronic control circuits were developed for use in high power low voltage circuit breakers. These control circuits performed functions such as instantaneous and delayed tripping which were traditionally achieved by magnetic and thermal means. The improved accuracy and flexibility of the solid state electronic controls resulted in their wide-spread acceptance, even though the electronic control circuits were more expensive than their mechanical counterparts.
The earliest electronic control circuit designs utilized discrete components such as transistors, resistors, and capacitors. More recent designs have included integrated circuits which have provided improved product performance at a slightly reduced cost.
As the cost of energy continues its rapid rise, there is increasing interest in effectively controlling the usage of electrical energy through the design of more sophisticated electrical distribution circuits. Therefore, there is required a circuit breaker providing a more complex analysis of electrical conditions on the circuit being protected and even greater capability for coordination with other breakers. As always, it is extremely desirable to provide this capability at the same or lower cost.
A useful feature of prior art circuit breakers employing solid-state control units was the ability to derive operating power from current transformers within the breaker, the transformers also being used in the protection circuitry of the breaker to supply load current data for comparison to the time-current trip characteristic. The dual role of the current transformer resulted in conflicting design requirements, the necessity to provide a reliable representation of load current over the required range of expected load currents limiting the amount of power which could usefully be drawn from the transformer. Typically, less than one watt of circuit power could thus be obtained.
As previously mentioned, it is desirable to interconnect circuit breakers by means of control lines to provide coordination between main, feeder, and branch circuits. However, direct wiring of control signals between breakers is undesirable due to noise and reliability problems. The use of optical isolators has heretofore been impractical because of the high power supply requirement, especially in those applications which involve a large number of interconnect lines between breakers. Therefore, interconnection systems generally required separate power supplies. It would thus be desirable to provide a circuit breaker having a system for interfacing with other breakers and associated apparatus which provides the required isolation, while at the same time having a power consumption sufficiently low to be supplied by the sensing current transformer.